A power semiconductor device formed in a SiC semiconductor substrate, which has a larger property value than Si, can achieve a greater performance than a power semiconductor device formed in Si. Specifically, because SiC can function as semiconductor to a high temperature due to a wide energy gap (about 3 times wider than Si), SiC can have a high breakdown voltage due to a high dielectric breakdown voltage (about 10 times higher than Si), and SiC has an excellent radiation performance due to high thermal conductivity (about 3 times higher than Si), SiC can achieve higher current.
This kind of SiC semiconductor substrate has polar faces of Si-face ((0001) Si-face) and C-face ((000-1) C-face), and properties depend on the polar faces. Thus, when a device is formed using a SiC semiconductor substrate, a choice of polar face becomes important.
The polar face means a surface which is an ideal surface without defects and on which exposure probabilities of atoms constituting of a compound semiconductor (Si and C in a case of SiC) are not equal to each other. In other words, on a C-face in a SiC semiconductor substrate, mainly C is exposed from a substrate surface and an exposure probability of C is higher than Si, and on a SiC-face, mainly Si is exposed from a substrate surface and an exposure probability of Si is higher than C.
For example, in a case of SiC semiconductor substrate, a C-face has a lower contact resistance than a Si-face (for example, see Patent Document No. 1), and a C-face has a higher thermal oxidation rate than a Si-face (for example, see Patent Document No. 2). Furthermore, in a case where a MOSFET and the like is formed on a C-face, a channel mobility is high (for example, see Nonpatent Document No. 1).
The same as a SiC semiconductor substrate holds true for a gallium nitride (hereafter, referred to as GaN) semiconductor substrate. Specifically, a GaN semiconductor substrate has polar faces of a Ga-face and a N-face. In a GaN semiconductor substrate, it is known that growth on a Ga-face provides a high crystal quality, and it is general to form a device using a Ga-face as a front surface so that a high-quality epitaxial layer can be formed (for example, see Patent Document No. 3). On the other hand, in a GaN semiconductor substrate, a Ga-face has a lower contact resistance than a N-face. Thus, it is preferable that a rear surface is a Ga-face in order to reduce a contact resistance with a rear electrode (for example, see Nonpatent Document No. 2).